Malignant melanoma is one of the fastest rising cancers in the US. While treatment for clinical stage I and II melanoma involves wide local excision of the primary tumor, with staging of the regional lymph nodes by sentinel lymph node (SLN) biopsy, SLN biopsy is fraught with false-negative results. Technology that would reliably detect nodal metastases, not just the first draining node, could improve the sensitivity of melanoma nodal staging. Therefore, our objective is to (1) develop, characterize, validate and compare (A) optoacoustic ultra-acidic pH-responsive acidic pH targeted dye, pHO dye, and (B) external cell membrane anchored, V7- pHO, for tumor detection by multispectral optoacoustic tomography (MSOT) and (2) provide real-time color maps of tissue pH to allow for identification of regions of tumor in lymph nodes. Based on our preliminary data, we reason that it is possible to use our targeting methodology to identify very small metastases, which would allow complete lymph node dissection to be performed only for the 15% to 20% of patients who actually have cancer in the non-sentinel nodes. We hypothesize that pHO and V7-pHO dyes will facilitate detection of melanoma within lymph nodes and differentiate it from non-malignant, fibrous, or inflammatory tissue with high sensitivity and specificity using multispectral optoacoustic tomography. To test our hypothesis, we propose the following aims: 1) characterize and validate pHO and V7-pHO dyes to target acidic pHe for detection of tumor cells in vitro and in tissue phantoms; 2) assess pHO and V7-pHO dyes to facilitate detection of melanoma from non-malignant tissue in vivo using multispectral optoacoustic tomography; and 3) develop ratiometric algorithms based upon the spectral modulation of the pHO and V7-pHO dyes to produce real-time pHe measurement color maps and bi-color maps of tumors. Successful identification of melanoma containing lymph nodes using pHO or V7-pHO dyes detected using MSOT has potential to stratify patients for treatment in the clinic. Identification of metastases in lymph nodes or elsewhere in melanoma patients has potential for game- changing clinical management of patients with melanoma.